Differences in mRNA levels in Anabaena living freely or in symbiotic association with Azolla.

Azolla is a small water fern in whose leaf cavities the filamentous nitrogen-fixing cyanobacterium Anabaena azollae is symbiotically associated. Using cloned genes from Anabaena 7120 for glutamine synthetase (GS), ribulose-1,5-bisphosphate (RuBP) carboxylase, nitrogenase and the 32-kd protein of photosystem II, mRNA levels of the corresponding genes in the Anabaena endosymbiont were studied by Northern hybridization. In RNA isolated from the endosymbiont there is a 10-fold reduction of GS transcript levels, a greater than 5-fold increase in 32-kd transcript levels and a greater than 5-fold decrease in RuBP carboxylase transcript levels, compared with levels in the free-living Anabaena azollae. In the endosymbiont and in heterocysts of the free-living Anabaena azollae the nif H, nif D, and nif K genes are transcribed from a single nif HDK operon.